This invention relates to a process for improving the corrosion resistance of anodized metal surfaces, and particularly surfaces of aluminum metal that have been anodized to form an aluminum oxide layer.
Many of the agents used in the present invention are also disclosed for improving the drop-wise condensation properties of conversion coated metal in my U.S. Pat. No. 3,899,366 entitled "Treated Substrate For The Formation Of Drop-Wise Condensates And The Process For Preparing Same," which is incorporated herein by reference.
Anodized metal surfaces differ from conversion coated metal surfaces in both structure and suitability for various applications. As described in my U.S. Pat. No. 3,899,366, the conversion-coated metals are protected by a coating including chromium which is applied by a non-electrolytic process. In general, such coatings include chromium and a fraction of the underlying metal which, together, are more inert to chemical oxidation than the original metal. As disclosed in that patent, the coating with a fluorochemical improves the formation of drop-wise rather than film condensation on such surfaces which is particularly valuable in heat transfer surfaces such as cooling coils. It also improves corrosion resistance.
The sealing of anodized aluminum involves converting surface oxides such as Al.sub.2 O.sub.3 to their hydrates. A similar process is sometimes used with magnesium, titanium and some other metals.
The present invention is concerned with anodized surfaces, and particularly anodized aluminum surfaces, which have different structures, properties and applications than conversion coated surfaces.
In general, conversion coated surfaces are known to be used in applications where brightness or color is not a factor or, if coloring is desired, may be painted over. Anodized coatings on aluminum may have color impregnated thereon before or during sealing with boiling water.
U.S. Pat. No. 3,012,917 relates to a method of treating anodically oxidized aluminum to provide a sealing or protective coating and utilizes chlorofluorinated carboxylic acids and/or salts of a polyfluorinated carboxylic acid. British patent specification No. 1,054,316 discloses other processes for sealing freshly anodized aluminum.
In addition, it is well known to provide fluorinated polymeric coatings to metal surfaces as for example polytetrafluoroethylene on non-stick cookware, corrosion-resistant chemical vessels and the like.